Phonon-Induced Quantum Magnetic Deflagration in Mn12

A comprehensive set of experiments on the effect of high-frequency surface acoustic waves, SAWs, in the spin relaxation in Mn12-acetate is presented. We have studied the quantum magnetic deflagration induced by SAWs under various experimental conditions extending the data shown in a very recent paper [A. Hernandez-Minguez et. al., Phys. Rev. Lett. 95, 217205 (2005)]. We have focused our study on the dependence of both the ignition time and the propagation speed of the magnetic avalanches on the frequency, amplitude, and duration of the SAW pulses in experiments performed under different temperatures and external magnetic fields.Comment: 7 pages, 9 figure

Low energy magnetic excitations of the Mn_{12}-acetate spin cluster observed by neutron scattering

We performed high resolution diffraction and inelastic neutron scattering measurements of Mn_{12}-acetate. Using a very high energy resolution, we could separate the energy levels corresponding to the splitting of the lowest S multiplet. Data were analyzed within a single spin model (S=10 ground state), using a spin Hamiltonian with parameters up to 4^{th} order. The non regular spacing of the transition energies unambiguously shows the presence of high order terms in the anisotropy (D= -0.457(2) cm^{-1}, B_4^0 = -2.33(4) 10^{-5}cm^{-1}). The relative intensity of the lowest energy peaks is very sensitive to the small transverse term, supposed to be mainly responsible for quantum tunneling. This allows an accurate determination of this term in zero magnetic field (B_4^4 = \pm 3.0(5) 10^{-5} cm^{-1}). The neutron results are discussed in view of recent experiments and theories.Comment: 4 pages ? 3 figures, submitted to Physical Review Lette

High frequency resonant experiments in Fe8_8 molecular clusters

Precise resonant experiments on Fe$_{8}$ magnetic clusters have been conducted down to 1.2 K at various tranverse magnetic fields, using a cylindrical resonator cavity with 40 different frequencies between 37 GHz and 110 GHz. All the observed resonances for both single crystal and oriented powder, have been fitted by the eigenstates of the hamiltonian ${\cal H}=-DS_z^2+ES_x^2-g\mu_B{\bf H}\cdot {\bf S}$. We have identified the resonances corresponding to the coherent quantum oscillations for different orientations of spin S = 10.Comment: to appear in Phys.Rev. B (August 2000

Magnetic Field Effects on the Far-Infrared Absorption in Mn_12-acetate

We report the far-infrared spectra of the molecular nanomagnet Mn_12-acetate (Mn_12) as a function of temperature (5-300 K) and magnetic field (0-17 T). The large number of observed vibrational modes is related to the low symmetry of the molecule, and they are grouped together in clusters. Analysis of the mode character based on molecular dynamics simulations and model compound studies shows that all vibrations are complex; motion from a majority of atoms in the molecule contribute to most modes. Three features involving intramolecular vibrations of the Mn_12 molecule centered at 284, 306 and 409 cm-1 show changes with applied magnetic field. The structure near 284 cm$^{-1}$ displays the largest deviation with field and is mainly intensity related. A comparison between the temperature dependent absorption difference spectra, the gradual low-temperature cluster framework distortion as assessed by neutron diffraction data, and field dependent absorption difference spectra suggests that this mode may involve Mn motion in the crown.Comment: 5 pages, 4 figures, PRB accepte

Spin dynamics of Mn12-acetate in the thermally-activated tunneling regime: ac-susceptibility and magnetization relaxation

In this work, we study the spin dynamics of Mn12-acetate molecules in the regime of thermally assisted tunneling. In particular, we describe the system in the presence of a strong transverse magnetic field. Similar to recent experiments, the relaxation time/rate is found to display a series of resonances; their Lorentzian shape is found to stem from the tunneling. The dynamic susceptibility $\chi(w)$ is calculated starting from the microscopic Hamiltonian and the resonant structure manifests itself also in $\chi(w)$. Similar to recent results reported on another molecular magnet, Fe8, we find oscillations of the relaxation rate as a function of the transverse magnetic field when the field is directed along a hard axis of the molecules. This phenomenon is attributed to the interference of the geometrical or Berry phase. We propose susceptibility experiments to be carried out for strong transverse magnetic fields to study of these oscillations and for a better resolution of the sharp satellite peaks in the relaxation rates.Comment: 22 pages, 23 figures; submitted to Phys. Rev. B; citations/references adde

Detailed single crystal EPR lineshape measurements for the single molecule magnets Fe8Br and Mn12-ac

It is shown that our multi-high-frequency (40-200 GHz) resonant cavity technique yields distortion-free high field EPR spectra for single crystal samples of the uniaxial and biaxial spin S = 10 single molecule magnets (SMMs) [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O and [Fe8O2(OH)12(tacn)6]Br8.9H2O. The observed lineshapes exhibit a pronounced dependence on temperature, magnetic field, and the spin quantum numbers (Ms values) associated with the levels involved in the transitions. Measurements at many frequencies allow us to separate various contributions to the EPR linewidths, including significant D-strain, g-strain and broadening due to the random dipolar fields of neighboring molecules. We also identify asymmetry in some of the EPR lineshapes for Fe8, and a previously unobserved fine structure to some of the EPR lines for both the Fe8 and Mn12 systems. These findings prove relevant to the mechanism of quantum tunneling of magnetization in these SMMs.Comment: Phys. Rev. B, accepted with minor revision

Cytomolecular identification of individual wheat-wheat chromosome arm associations in wheat-rye hybrids

Chromosome pairing in the meiotic metaphase I of wheatrye hybrids has been characterized by sequential genomic and fluorescent in situ hybridization allowing not only the discrimination of wheat and rye chromosomes, but also the identification of the individual wheat and rye chromosome arms involved in the chromosome associations. The majority of associations (93.8%) were observed between the wheat chromosomes. The largest number of wheat-wheat chromosome associations (53%) was detected between the A and D genomes, while the frequency of B-D and A-B associations was significantly lower (32 and 8%, respectively). Among the A-D chromosome associations, pairing between the 3AL and 3DL arms was observed with the highest frequency, while the most frequent of all the chromosome associations (0.113/ cell) was found to be the 3DS-3BS. Differences in the pairing frequency of the individual chromosome arms of wheat-rye hybrids have been discussed in relation to the homoeologous relationships between the constituent genomes of hexaploid wheat

Semiconductive and Photoconductive Properties of the Single Molecule Magnets Mn12_{12}-Acetate and Fe8_8Br8_8

Resistivity measurements are reported for single crystals of Mn$_{12}$-Acetate and Fe$_8$Br$_8$. Both materials exhibit a semiconductor-like, thermally activated behavior over the 200-300 K range. The activation energy, $E_a$, obtained for Mn$_{12}$-Acetate was 0.37 $\pm$ 0.05 eV, which is to be contrasted with the value of 0.55 eV deduced from the earlier reported absorption edge measurements and the range of 0.3-1 eV from intramolecular density of states calculations, assuming $2E_a$= $E_g$, the optical band gap. For Fe$_8$Br$_8$, $E_a$ was measured as 0.73 $\pm$ 0.1 eV, and is discussed in light of the available approximate band structure calculations. Some plausible pathways are indicated based on the crystal structures of both lattices. For Mn$_{12}$-Acetate, we also measured photoconductivity in the visible range; the conductivity increased by a factor of about eight on increasing the photon energy from 632.8 nm (red) to 488 nm (blue). X-ray irradiation increased the resistivity, but $E_a$ was insensitive to exposure.Comment: 7 pages, 8 figure

Specific heat and heat conductivity of the BaTiO3 polycrystalline films with the thickness in the range 20 - 1100 nm

Thermal properties - specific heat and heat conductivity coefficient - of polycrystalline BaTiO3 films on massive substrates were studied as a function of the temperature and the film thickness by ac-hot probe method. The anomalies of specific heat with decreasing of the film thickness from 1100 to 20 nm revealed the reducing of critical temperature (Tc) and excess entropy of the ferroelectric phase transition, which becomes diffused. The critical thickness of the film at which Tc = 0 estimated as 2.5 nm.Comment: 12 pages, 7 figures, 2 tables, 450kb; submitted to J.Phys.:Cond.Mat